Refactor: Rename DocumentOperator to DocumentService

Updated project and solution files to rename and restructure
`DocumentOperator` to `DocumentService`. Modified project
references in `DocumentService.API.csproj`, `DocumentService.Application.csproj`,
and `DocumentService.Infrastructure.csproj` to reflect the new
naming convention. Updated `DocumentService.sln` to remove
old project references, add new ones, and adjust configuration
and nested project mappings accordingly. This change aligns
the project structure with the new naming convention.

DocumentService.Domain/Common/Exceptions/NotFoundException.cs

@@ -0,0 +1,22 @@
+using System.Runtime.Serialization;
+
+namespace DocumentService.Domain.Common.Exceptions;
+
+/// <summary>
+/// Exception thrown when a requested resource is not found.
+/// Maps to HTTP 404 Not Found in the API layer.
+/// </summary>
+public class NotFoundException : Exception
+{
+    public NotFoundException()
+    {
+    }
+
+    public NotFoundException(string? message) : base(message)
+    {
+    }
+
+    public NotFoundException(string? message, Exception? innerException) : base(message, innerException)
+    {
+    }
+}